Solid Electrolytic Capacitor Containing a Pre-Coat Layer

ABSTRACT

A solid electrolytic capacitor that contains an anode body formed from an electrically conductive powder, dielectric located over and/or within the anode body, an adhesion coating overlying the dielectric, and a solid electrolyte overlying the adhesion coating is provided. The powder has a high specific charge and in turn a relative dense packing configuration. Despite being formed from such a powder, the present inventors have discovered that the conductive polymer can be readily impregnated into the pores of the anode. This is accomplished, in part, through the use of a discontinuous precoat layer in the adhesion coating that overlies the dielectric. The precoat layer contains a plurality of discrete nanoprojections of a manganese oxide (e.g., manganese dioxide).

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application Ser. No. 61/822,508 (filed on May 13, 2013) and which is incorporated herein in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors (e.g., tantalum capacitors) have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Conventional solid electrolytic capacitors may be formed by pressing a metal powder (e.g., tantalum) around a metal lead wire, sintering the pressed part, anodizing the sintered anode, and thereafter applying a solid electrolyte. Intrinsically conductive polymers are often employed as the solid electrolyte due to their advantageous low equivalent series resistance (“ESR”) and “non-burning/non-ignition” failure mode. Such electrolytes can be formed through in situ polymerization of the monomer in the presence of a catalyst and dopant. Alternative, premade conductive polymer slurries may also be employed. Regardless of how they are formed, one problem with conductive polymer electrolytes is that it is difficult for such polymers to penetrate and uniformly coat the pores of the anode. Not only does this reduce the points of contact between the electrolyte and dielectric, but it can also cause facilitate delamination of the polymer from the dielectric during mounting or use. As a result of these problems, it is often difficult to achieve ultralow ESR and/or leakage current values in conventional conductive polymer capacitors. The problems are compounded when the valve metal powder used to form the anode has a high specific charge (e.g., about 70,000 microFarads*Volts per gram (“μF*V/g”) or more), which is desired for achieving high capacitance values. Such high “CV/g” powders are generally formed from particles having a small size and large surface area, which results in the formation of small pores between the particles that are even more difficult to impregnate with the conductive polymer.

As such, a need currently exists for an improved electrolytic capacitor containing a conductive polymer solid electrolyte.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a solid electrolytic capacitor is disclosed that comprises an anode body formed from an electrically conductive powder, wherein the powder has a specific charge of about 70,000 μF*V/g or more. A dielectric overlies the anode body, an adhesion coating overlies the dielectric, and solid electrolyte overlies the dielectric. The adhesion coating includes a discontinuous precoat layer and contains a plurality of discrete nanoprojections of a manganese oxide, and the solid electrolyte includes a conductive polymer layer.

In accordance with another embodiment of the present invention, a method for forming a solid electrolytic capacitor is disclosed. The method comprises contacting an anode that contains an anode body and a dielectric with a solution that contains a manganese oxide precursor, wherein the anode body is formed from an electrically conductive powder having a specific charge of about 70,000 μF*V/g or more. The precursor is pyrolytically converted to form a discontinuous layer containing a plurality of discrete nanoprojections of a manganese oxide. A conductive polymer layer is formed that contacts the nanoprojections and the dielectric.

Other features and aspects of the present invention are set forth in greater detail below.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended FIGURE in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor that may be formed in accordance with the present invention.

Repeat use of references characters in the present specification and drawing is intended to represent same or analogous features or elements of the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary construction.

Generally speaking, the present invention is directed to a solid electrolytic capacitor that contains an anode body formed from an electrically conductive powder, dielectric located over and/or within the anode body, an adhesion coating overlying the dielectric, and a solid electrolyte overlying the adhesion coating. The powder has a high specific charge and in turn a relative dense packing configuration. Despite being formed from such a powder, the present inventors have discovered that the conductive polymer can be readily impregnated into the pores of the anode. This is accomplished, in part, through the use of a discontinuous precoat layer in the adhesion coating that overlies the dielectric. The precoat layer contains a plurality of discrete nanoprojections of a manganese oxide (e.g., manganese dioxide). Without intending to be limited by theory, it is believed that the small size of the discrete nanoprojections allows them to more readily penetrate into the small pores of the anode body than would otherwise be possible with a conventional conductive polymer. When deposited, the nanoprojections can also become embedded into the conductive polymer layer as it is formed, which can enhance adhesion between the dielectric and the conductive polymer layer. The improved adhesion reduces the likelihood of delamination and also allows a greater amount of the polymer to penetrate into the anode, thus improving capacitance while also minimizing leakage current and ESR. Because the precoat layer is formed as discrete nanoprojections rather than as a continuous layer, the conductive polymer may also be able to contact a substantial portion of the dielectric, whether direction or through another layer. The relatively large degree of direct contact between the conductive polymer and dielectric can even further reduce ESR.

Various embodiments of the invention will now be described in more detail.

I. Anode.

As indicated above, the anode may be formed from a powder having a high specific charge. That is, the powder may have a specific charge of about 70,000 microFarads*Volts per gram (“μF*V/g”) or more, in some embodiments about 80,000 μF*V/g or more, in some embodiments about 90,000 μF*V/g or more, in some embodiments from about 100,000 to about 350,000 μF*V/g, and in some embodiments, from about 120,000 to about 250,000 μF*V/g.

The powder may contain individual particles and/or agglomerates of such particles. Compounds for forming the powder include a valve metal (i.e., metal that is capable of oxidation) or valve metal-based compound, such as tantalum, niobium, aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitrides thereof, and so forth. For example, the valve metal composition may contain an electrically conductive oxide of niobium, such as niobium oxide having an atomic ratio of niobium to oxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments 1:1.0±0.1, and in some embodiments, 1:1.0±0.05. For example, the niobium oxide may be NbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂. Examples of such valve metal oxides are described in U.S. Pat. No. 6,322,912 to Fife; U.S. Pat. No. 6,391,275 to Fife et al.; U.S. Pat. No. 6,416,730 to Fife et al.; U.S. Pat. No. 6,527,937 to Fife; U.S. Pat. No. 6,576,099 to Kimmel, et al.; U.S. Pat. No. 6,592,740 to Fife, et al.; and U.S. Pat. No. 6,639,787 to Kimmel, et al.; and U.S. Pat. No. 7,220,397 to Kimmel, al., as well as U.S. Patent Application Publication Nos. 2005/0019581 to Schnitter; 2005/0103638 to Schnitter, et al.; 2005/0013765 to Thomas, et al.

The apparent density (or Scott density) of the powder may vary as desired, but typically ranges from about 1 to about 8 grams per cubic centimeter (g/cm³), in some embodiments from about 2 to about 7 g/cm³, and in some embodiments, from about 3 to about 6 g/cm³. To achieve the desired level of packing and apparent density, the size and shape of the particles (or agglomerates) may be carefully controlled. For example, the shape of the particles may be generally spherical, nodular, etc. The particles may have an average size of from about 0.1 to about 20 micrometers, in some embodiments from about 0.5 to about 15 micrometers, and in some embodiments, from about 1 to about 10 micrometers.

The powder may be formed using techniques known to those skilled in the art. A precursor tantalum powder, for instance, may be formed by reducing a tantalum salt (e.g., potassium fluotantalate (K₂TaF₇), sodium fluotantalate (Na₂TaF₇), tantalum pentachloride (TaCl₅), etc.) with a reducing agent (e.g., hydrogen, sodium, potassium, magnesium, calcium, etc.). Such powders may be agglomerated in a variety of ways, such as through one or multiple heat treatment steps at a temperature of from about 700° C. to about 1400° C., in some embodiments from about 750° C. to about 1200° C., and in some embodiments, from about 800° C. to about 1100° C. Heat treatment may occur in an inert or reducing atmosphere. For example, heat treatment may occur in an atmosphere containing hydrogen or a hydrogen-releasing compound (e.g., ammonium chloride, calcium hydride, magnesium hydride, etc.) to partially sinter the powder and decrease the content of impurities (e.g., fluorine). If desired, agglomeration may also be performed in the presence of a getter material, such as magnesium. After thermal treatment, the highly reactive coarse agglomerates may be passivated by gradual admission of air. Other suitable agglomeration techniques are also described in U.S. Pat. No. 6,576,038 to Rao; U.S. Pat. No. 6,238,456 to Wolf, et al.: U.S. Pat. No. 5,954,856 to Pathare, et al.; U.S. Pat. No. 5,082,491 to Rerat; U.S. Pat. No. 4,555,268 to Getz; U.S. Pat. No. 4,483,819 to Albrecht, et al.; U.S. Pat. No. 4,441,927 to Getz, et al.; and U.S. Pat. No. 4,017,302 to Bates, et al.

The desired size and/or shape of the particles may be achieved by controlling various processing parameters as is known in the art, such as the parameters relating to powder formation (e.g., reduction process) and/or agglomeration (e.g., temperature, atmosphere, etc.). Milling techniques may also be employed to grind a precursor powder to the desired size. Any of a variety of milling techniques may be utilized to achieve the desired particle characteristics. For example, the powder may initially be dispersed in a fluid medium (e.g., ethanol, methanol, fluorinated fluid, etc.) to form a slurry. The slurry may then be combined with a grinding media (e.g., metal balls, such as tantalum) in a mill. The number of grinding media may generally vary depending on the size of the mill, such as from about 100 to about 2000, and in some embodiments from about 600 to about 1000. The starting powder, the fluid medium, and grinding media may be combined in any proportion. For example, the ratio of the starting powder to the grinding media may be from about 1:5 to about 1:50. Likewise, the ratio of the volume of the fluid medium to the combined volume of the starting powder may be from about 0.5:1 to about 3:1, in some embodiments from about 0.5:1 to about 2:1, and in some embodiments, from about 0.5:1 to about 1:1. Some examples of mills that may be used in the present invention are described in U.S. Pat. Nos. 5,522,558; 5,232,169; 6,126,097; and 6,145,765. Milling may occur for any predetermined amount of time needed to achieve the target size. For example, the milling time may range from about 30 minutes to about 40 hours, in some embodiments, from about 1 hour to about 20 hours, and in some embodiments, from about 5 hours to about 15 hours. Milling may be conducted at any desired temperature, including at room temperature or an elevated temperature. After milling, the fluid medium may be separated or removed from the powder, such as by air-drying, heating, filtering, evaporating, etc.

Various other conventional treatments may also be employed in the present invention to improve the properties of the powder. For example, in certain embodiments, the particles may be treated with sinter retardants in the presence of a dopant, such as aqueous acids (e.g., phosphoric acid). The amount of the dopant added depends in part on the surface area of the powder, but is typically present in an amount of no more than about 200 parts per million (“ppm”). The dopant may be added prior to, during, and/or subsequent to any heat treatment step(s).

The particles may also be subjected to one or more deoxidation treatments to improve ductility and reduce leakage current in the anodes. For example, the particles may be exposed to a getter material (e.g., magnesium), such as described in U.S. Pat. No. 4,960,471. The getter material may be present in an amount of from about 2% to about 6% by weight. The temperature at which deoxidation occurs may vary, but typically ranges from about 700° C. to about 1600° C., in some embodiments from about 750° C. to about 1200° C., and in some embodiments, from about 800° C. to about 1000° C. The total time of deoxidation treatment(s) may range from about 20 minutes to about 3 hours. Deoxidation also preferably occurs in an inert atmosphere (e.g., argon). Upon completion of the deoxidation treatment(s), the magnesium or other getter material typically vaporizes and forms a precipitate on the cold wall of the furnace. To ensure removal of the getter material, however, the fine agglomerates and/or coarse agglomerates may be subjected to one or more acid leaching steps, such as with nitric acid, hydrofluoric acid, etc.

To facilitate the construction of the anode, certain components may also be included in the powder. For example, the powder may be optionally mixed with a binder and/or lubricant to ensure that the particles adequately adhere to each other when pressed to form the anode body. Suitable binders may include, for instance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol); poly(vinyl pyrollidone); cellulosic polymers, such as carboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and methylhydroxyethyl cellulose; atactic polypropylene, polyethylene; polyethylene glycol (e.g., Carbowax from Dow Chemical Co.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, and polyacrylamides, high molecular weight polyethers; copolymers of ethylene oxide and propylene oxide; fluoropolymers, such as polytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefin copolymers; acrylic polymers, such as sodium polyacrylate, poly(lower alkyl acrylates), poly(lower alkyl methacrylates) and copolymers of lower alkyl acrylates and methacrylates; and fatty acids and waxes, such as stearic and other soapy fatty acids, vegetable wax, microwaxes (purified paraffins), etc. The binder may be dissolved and dispersed in a solvent. Exemplary solvents may include water, alcohols, and so forth. When utilized, the percentage of binders and/or lubricants may vary from about 0.1% to about 8% by weight of the total mass. It should be understood, however, that binders and/or lubricants are not necessarily required in the present invention.

The resulting powder may be compacted to form a pellet using any conventional powder press device. For example, a press mold may be employed that is a single station compaction press containing a die and one or multiple punches. Alternatively, anvil-type compaction press molds may be used that use only a die and single lower punch. Single station compaction press molds are available in several basic types, such as cam, toggle/knuckle and eccentric/crank presses with varying capabilities, such as single action, double action, floating die, movable platen, opposed ram, screw, impact, hot pressing, coining or sizing. The powder may be compacted around an anode lead (e.g., tantalum wire). It should be further appreciated that the anode lead may alternatively be attached (e.g., welded) to the anode body subsequent to pressing and/or sintering of the anode body.

After compaction, any binder/lubricant may be removed by heating the pellet under vacuum at a certain temperature (e.g., from about 150° C. to about 500° C.) for several minutes. Alternatively, the binder/lubricant may also be removed by contacting the pellet with an aqueous solution, such as described in U.S. Pat. No. 6,197,252 to Bishop, et al. Thereafter, the pellet is sintered to form a porous, integral mass. For example, in one embodiment, the pellet may be sintered at a temperature of from about 1200° C. to about 2000° C., and in some embodiments, from about 1500° C. to about 1800° C. under vacuum or an inert atmosphere. Upon sintering, the pellet shrinks due to the growth of bonds between the particles. The pressed density of the pellet after sintering may vary, but is typically from about 2.0 to about 7.0 grams per cubic centimeter, in some embodiments from about 2.5 to about 6.5, and in some embodiments, from about 3.0 to about 6.0 grams per cubic centimeter. The pressed density is determined by dividing the amount of material by the volume of the pressed pellet.

The anode may also have a relatively low carbon and oxygen content. For example, the anode may have no more than about 50 ppm carbon, and in some embodiments, no more than about 10 ppm carbon. Likewise, the anode may have no more than about 0.15 ppm/pC/g oxygen, and in some embodiments, no more than about 0.10 ppm/pC/g oxygen. Oxygen content may be measured by LECO Oxygen Analyzer and includes oxygen in natural oxide on the tantalum surface and bulk oxygen in the tantalum particles. Bulk oxygen content is controlled by period of crystalline lattice of tantalum, which is increasing linearly with increasing oxygen content in tantalum until the solubility limit is achieved. This method was described in “Critical Oxygen Content In Porous Anodes Of Solid Tantalum Capacitors”, Pozdeev-Freeman et al., Journal of Materials Science: Materials In Electronics 9, (1998) 309-311 wherein X-ray diffraction analysis (XRDA) was employed to measure period of crystalline lattice of tantalum. Oxygen in sintered tantalum anodes may be limited to thin natural surface oxide, while the bulk of tantalum is practically free of oxygen.

Although not required, the thickness of the anode may be selected to improve the electrical performance of the capacitor. For example, the thickness of the anode may be about 4 millimeters or less, in some embodiments, from about 0.05 to about 2 millimeters, and in some embodiments, from about 0.1 to about 1 millimeter. The shape of the anode may also be selected to improve the electrical properties of the resulting capacitor. For example, the anode may have a shape that is curved, sinusoidal, rectangular, U-shaped, V-shaped, etc. The anode may also have a “fluted” shape in that it contains one or more furrows, grooves, depressions, or indentations to increase the surface to volume ratio to minimize ESR and extend the frequency response of the capacitance. Such “fluted” anodes are described, for instance, in U.S. Pat. No. 6,191,936 to Webber, et al.; U.S. Pat. No. 5,949,639 to Maeda, et al.; and U.S. Pat. No. 3,345,545 to Bourgault et al., as well as U.S. Patent Application Publication No. 2005/0270725 to Hahn, et al.

II. Dielectric

The anode is also coated with a dielectric. The dielectric may be formed by anodically oxidizing (“anodizing”) the sintered anode so that a dielectric layer is formed over and/or within the anode. For example, a tantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅). Typically, anodization is performed by initially applying a solution to the anode, such as by dipping anode into the electrolyte. A solvent is generally employed, such as water (e.g., deionized water). To enhance ionic conductivity, a compound may be employed that is capable of dissociating in the solvent to form ions. Examples of such compounds include for instance, acids, such as described below with respect to the electrolyte. For example, an acid (e.g., phosphoric acid) may constitute from about 0.01 wt. % to about 5 wt. %, in some embodiments from about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, from about 0.1 wt. % to about 0.5 wt. % of the anodizing solution. If desired, blends of acids may also be employed.

A current is passed through the anodizing solution to form the dielectric layer. The value of the formation voltage manages the thickness of the dielectric layer. For example, the power supply may be initially set up at a galvanostatic mode until the required voltage is reached. Thereafter, the power supply may be switched to a potentiostatic mode to ensure that the desired dielectric thickness is formed over the entire surface of the anode. Of course, other known methods may also be employed, such as pulse or step potentiostatic methods. The voltage at which anodic oxidation occurs typically ranges from about 4 to about 250 V, and in some embodiments, from about 9 to about 200 V, and in some embodiments, from about 20 to about 150 V. During oxidation, the anodizing solution can be kept at an elevated temperature, such as about 30° C. or more, in some embodiments from about 40° C. to about 200° C., and in some embodiments, from about 50° C. to about 100° C. Anodic oxidation can also be done at ambient temperature or lower. The resulting dielectric layer may be formed on a surface of the anode and within its pores.

If desired, each stage of anodization may be repeated for one or more cycles to achieve the desired dielectric thickness. Furthermore, the anode may also be rinsed or washed with another solvent (e.g., water) after the first and/or second stages to remove the electrolyte.

III. Adhesion Coating

As indicated above, the adhesion coating of the capacitor contains a discontinuous precoat layer, which includes a plurality of discrete nanoprojections of a manganese oxide (e.g., manganese dioxide) that can penetrate into the small pores of the anode body and ultimately become embedded into the conductive polymer layer. Because the precoat layer is formed as discrete nanoprojections rather than as a continuous layer, the conductive polymer may be able to directly contact a substantial portion of the dielectric, either directly or through contact with another layer, such as described below. The relatively large degree of contact between the conductive polymer and dielectric can even further reduce ESR. To accomplish the desired result without adversely impacting the overall performance of the capacitor, the average size (e.g., diameter) of the nanoprojections is typically large enough so that an improvement in adhesion is achieved, but yet not so large that they are incapable of penetrating into the pores of the anode. In this regard, the nanoprojections typically have an average size of from about 5 nanometers to about 500 nanometers, in some embodiments from about 6 nanometers to about 250 nanometers, in some embodiments, from about 8 nanometers to about 150 nanometers, and in some embodiments, from about 10 nanometers to about 110 nanometers. The term “average diameter” may, for example, refer to the average value for the major axis of the nanoprojections when viewed from above (the maximum diameter). Such diameters may be obtained, for example, using known techniques, such as photon correlation spectroscopy, dynamic light scattering, quasi-elastic light scattering, etc. Various particle size analyzers may be employed to measure the diameter in this manner. One particular example is a Corouan VASCO 3 Particle Size Analyzer. Although not necessarily required, the nanoprojections may also have a narrow size distribution, which may further improve the properties of the capacitor. For instance, about 50% or more, in some embodiments about 70% or more, and in some embodiments, about 90% or more of the nanoprojections may have an average size within the ranges noted above. The number of nanoprojections having a certain size may be determined using the techniques noted above, wherein the percent volume can be correlated to the number of particles having a certain absorbance unit (“au”).

In addition to their size, the surface coverage of the nanoprojections on the dielectric may also be selectively controlled to help achieve the desired electrical performance. That is, too small of a surface coverage may limit the ability to the conductive polymer layer to better adhere to the dielectric, but too large of a coverage may adversely impact the ESR of the capacitor. In this regard, the surface coverage of the nanoprojections is typically from about 0.1% to about 40%, in some embodiments from about 0.5% to about 30%, and in some embodiments, from about 1% to about 20%. The degree of surface coverage may be calculated in a variety of ways, such as by dividing the “actual capacitance” value by the “normal capacitance” value and then multiplying by 100. The “normal capacitance” is determined after forming the nanoprojections and then impregnating the anode with the conductive polymer solution, while the “actual capacitance” is determined after forming the nanoprojections, impregnating the anode with the conductive polymer solution, washing the conductive polymer solution from the interior of the anode, and then drying the anode to remove moisture.

A variety of different techniques may be employed to form the precoat layer of the present invention. As is known in the art, manganese oxides (e.g., manganese dioxide) are typically formed through pyrolytic decomposition of a precursor (e.g., manganese nitrate (Mn(NO₃)₂)), such as described in U.S. Pat. No. 4,945,452 to Sturmer, et al. For example, a dielectric-coated anode body may be contacted with a solution (e.g., dipped, immersed, sprayed, etc.) that contains the precursor and thereafter heated for conversion into the oxide. If desired, multiple application steps may be employed. The amount of time in which the anode body is in contact with a manganese oxide precursor solution may vary as desired. For example, the anode body may be dipped into such a solution for a period of time ranging from about 10 seconds to about 10 minutes.

The manganese oxide precursor solution may optionally contain a surfactant. Without intending to be limited by theory, it is believed that such a surfactant can reduce surface tension and thereby improve penetration of the solution into the interior of the anode body. Particularly suitable are nonionic surfactants, such as a polyglycol ether (e.g., polyoxyethylene alkyl ether), nonylphenoxypoly-(ethyleneoxy)ethanol (e.g., Igepal CO-630); isooctylphenoxy-polyethoxyethanol (e.g., Triton X-100), benzyletheroctylphenol-ethylene oxide condensate (e.g., Triton CF-10), 3,6-dimethyl-4-octyne-3,6-diol (e.g., Surfynol 82), and so forth. To achieve the desired improvement in the impregnation of the manganese oxide precursor without adversely impacting other characteristics of the capacitor, it is generally desired that the concentration of the surfactant is selectively controlled within a certain range. For example, the solution into which the anode body is dipped may contain the surfactant in an amount of from about 0.01 wt. % to about 30 wt. %, in some embodiments from about 0.05 wt. % to about 25 wt. %, and in some embodiments, from about 0.1 wt. % to about 20 wt. %. The precursor(s) (e.g., manganese nitrate) may likewise constitute from about 1 wt. % to about 55 wt. % in some embodiments from about 2 wt. % to about 15 wt. %, and in some embodiments, from about 5 wt. % to about 10 wt. %, of the solution. A carrier, such as water, may also be employed in the solution. Aqueous solutions of the present invention may, for instance, contain water in an amount of from about 30 wt. % to about 95 wt. %, in some embodiments from about 40 wt. % to about 99 wt. % and in some embodiments, from about 50 wt. % to about 95 wt. %. It should be understood that the actual amounts of the components in the solution may vary depending upon such factors as the particle size and distribution of particles in the anode, the temperature at which decomposition is performed, the identity of the dispersant, the identity of the carrier, etc.

If desired, the anode body may be contacted with a humidified atmosphere in a pretreatment step that occurs prior to contact with a manganese oxide precursor solution. Without intending to be limited by theory, the present inventors believe that the presence of a certain amount of water vapor can slow the thermal decomposition reaction of manganese dioxide, thereby causing it to form as dispersed nanoprojections. For example, during the pretreatment step, the anode body can be exposed to an atmosphere having a humidity level of from about 1 to about 30 grams of water per cubic meter of air (g/m³), in some embodiments from about 4 to about 25 g/m³, and in some embodiments, from about 5 to about 20 g/m³. The relative humidity may likewise range from about 30% to about 90%, in some embodiments from about 40% to about 85%, and in some embodiments, from about 50% to about 80%. The temperature of the humidified atmosphere may vary, such as from about 10° C. to about 50° C., in some embodiments from about 15° C. to about 45° C., and in some embodiments, from about 20° C. to about 40° C. In addition to a pretreatment step, the anode body may also be contacted with a humidified atmosphere in an intermediate treatment step, which occurs after contact with a manganese oxide precursor solution. The humidified atmosphere in the intermediate treatment step may have the same or different conditions than that of the pretreatment step, but is generally within the ranges noted above.

Regardless, once contacted with the precursor solution for the desired amount of time, the part is heated to a temperature sufficient to pyrolytically convert the precursor (e.g., manganese nitrate) to an oxide. Heating may occur, for instance, in a furnace at a temperature of from about 150° C. to about 300° C., in some embodiments from about 180° C. to about 290° C., and in some embodiments, from about 190° C. to about 260° C. Heating may be conducted in a moist or dry atmosphere. In certain embodiments, for instance, heating may be conducted in a humidified atmosphere, which may be the same or different than the atmospheres used in the aforementioned pretreatment and intermediate treatment steps, but generally within the conditions noted above. The time for the conversion depends on the furnace temperature, heat transfer rate and atmosphere, but generally is from about 3 to about 5 minutes. After pyrolysis, the leakage current may sometimes be high due to damage suffered by the dielectric film during the deposition of the manganese dioxide. To reduce this leakage, the capacitor may be reformed in an anodization bath as is known in the art. For example, the capacitor may be dipped into an electrolyte such as described above and then subjected to a DC current.

If desired, the adhesive coating may also contain other layers to help reduce the likelihood of delamination. In one embodiment, for example, the adhesive coating may include a resinous layer, which may be continuous or discontinuous in nature. When employed, the particular arrangement of the resinous layer relative to the precoat layer may vary as desired. In one embodiment, for instance, the precoat layer may be initially formed on the dielectric, and the resinous layer may thereafter be applied to the coated dielectric. In such embodiments, the precoat layer overlies the dielectric and the resinous layer overlies the precoat layer and may contact the precoat layer and/or the dielectric. Despite the presence of the resinous layer, it is believed that the coated nanoprojections of the precoat layer are still capable of becoming embedded within the conductive polymer layer. In another embodiment, the resinous layer may be initially applied to the dielectric, and the precoat layer may thereafter be formed thereon. In such embodiments, the resinous layer overlies the dielectric and the precoat layer overlies the resinous layer.

The resinous layer may generally include a natural or synthetic resin, which may be a solid or semi-solid material that is polymeric in nature or capable of being polymerized, cured, or otherwise hardened. It is also typically desired that the resin is relatively insulative in nature. As used herein, the term “relatively insulative” generally means more resistive than the conductive polymer that primarily forms the conductive polymer layer. For example, in some embodiments, a relatively insulative resin can have a resistivity at 20° C. of about 1000 Ω-cm or more, in some embodiments about 10,000 Ω-cm or more, in some embodiments about 1×10⁵ Ω-cm or more, and in some embodiments, about 1×10¹⁰ Ω-cm or more. Some examples of suitable resins that may be employed include, but are not limited to, polyurethane, polystyrene, esters of unsaturated or saturated fatty acids (e.g., glycerides), and so forth. For instance, suitable esters of fatty acids include, but are not limited to, esters of lauric acid, myristic acid, palmitic acid, stearic acid, eleostearic acid, oleic acid, linoleic acid, linolenic acid, aleuritic acid, shellolic acid, and so forth. These esters of fatty acids have been found particularly useful when used in relatively complex combinations to form a “drying oil”, which allows the resulting film to rapidly polymerize into a stable layer. Such drying oils may include mono-, di-, and/or tri-glycerides, which have a glycerol backbone with one, two, and three, respectively, fatty acyl residues that are esterified. For instance, some suitable drying oils that may be used include, but are not limited to, olive oil, linseed oil, castor oil, tung oil, soybean oil, and shellac. Shellac, which is believed to contain esters of various aliphatic and alicyclic hydroxy acids (e.g., aleuritic acid and shellolic acid), is particularly suitable. These and other resin materials are described in more detail in U.S. Pat. No. 6,674,635 to Fife, et al.

When employed, the esters of fatty acids, such as described above, may exist naturally or be refined from natural materials. For example, soybean oil is often obtained from soybeans through refinement by solvent extraction with petroleum hydrocarbons or using continuous screw press operations. Upon extraction, the obtained soybean oil is primarily constituted of triglycerides of oleic acid, linoleic acid, and linolenic acid. Tung oil, on the other hand, is a drying oil that often requires no such refinement. In some instances, it may be desired to initiate further esterification of a fatty acid mixture by reacting an alcohol therewith. Such fatty acid/alcohol ester derivatives may generally be obtained using any known alcohol capable of reacting with a fatty acid. For example, in some embodiments, monohydric and/or polyhydric alcohols with less than 8 carbon atoms, and in some embodiments, less than 5 carbon atoms, may be used in the present invention. Specific embodiments of the present invention include the use of methanol, ethanol, butanol, as well as various glycols, such as propylene glycol, hexylene glycol, etc. In one particular embodiment, shellac can be esterified by mixing it with an alcohol, such as described above. Specifically, shellac is a resinous excretion of an insect that is believed to contain a complex mixture of fatty acids that, to some extent, are esterified. Thus, when mixed with an alcohol, the fatty acid groups of the shellac are further esterified by reaction with the alcohol.

A resinous layer can be formed in a variety of different ways. For example, in one embodiment, the anode can be dipped into a solution of the desired resin(s). The solution can be formed by dissolving the selected protective resin into a solvent, such as water or a non-aqueous solvent. Some suitable non-aqueous solvents can include, but are not limited to, methanol, ethanol, butanol, as well as various glycols, such as propylene glycol, hexylene glycol, di(ethylene acetate) glycol, etc. Particularly desired non-aqueous solvents are those having a boiling point greater than about 80° C., in some embodiments greater than about 120° C., and in some embodiments, greater than about 150° C. As described above, the formation of a solution using a non-aqueous solvent may also lead to further esterification of fatty acids when such resinous materials are utilized. The anode can be dipped into the solution one or more times, depending on the desired thickness. For example, in some embodiments, multiple resinous layers may be employed, such as 2 to 10 layers, and in some embodiments, from 3 to 7 layers. Each layer may have a target thickness of, for instance, about 100 nanometers or less, in some embodiments about 30 nanometers or less, and in some embodiments, about 10 nanometers or less. Besides dipping, it should also be understood that other conventional application methods, such as sputtering, screen printing, electrophoretic coating, electron beam deposition, vacuum deposition, spraying, and the like, can also be used.

After forming the resinous layer, the anode part may be heated or otherwise cured. Heating can facilitate evaporation of any solvent used during application, and may also aid in the esterification and/or polymerization of the resinous materials. To facilitate esterification and/or polymerization, curing agents may also be added to the resinous layer. For instance, one example of a curing agent that can be used with shellac is sulfuric acid. The time and temperature at which heating occurs generally varies depending on the specific resinous materials utilized. Typically, each layer is dried at a temperature ranging from about 30° C. to about 300° C., and in some embodiments, from about 50° C. to about 150° C., for a time period ranging from about 1 minute to about 60 minutes, and in some embodiments, from about 15 minutes to about 30 minutes. It should also be understood that heating need not be utilized after application of each resinous layer.

IV. Solid Electrolyte

As noted above, the solid electrolyte includes a conductive polymer layer that overlies the adhesion coating and dielectric. The conductive polymer is typically Tr-conjugated and has electrical conductivity after oxidation or reduction, such as an electrical conductivity of at least about 1 μS/cm. Examples of such Tr-conjugated conductive polymers include, for instance, polyheterocycles (e.g., polypyrroles, polythiophenes, polyanilines, etc.), polyacetylenes, poly-p-phenylenes, polyphenolates, and so forth. In one embodiment, for example, the polymer is a substituted polythiophene, such as those having the following general structure:

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g., methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is a linear or branched, optionally substituted C₁ to C₁₈ alkyl radical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl cyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g., phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkyl radical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkyl radical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and in one embodiment, 0; and

n is from 2 to 5,000, in some embodiments from 4 to 2,000, and in some embodiments, from 5 to 1,000. Example of substituents for the radicals “D” or “R₇” include, for instance, alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone, sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups, carboxylamide groups, and so forth.

Particularly suitable thiophene polymers are those in which “D” is an optionally substituted C₂ to C₃ alkylene radical. For instance, the polymer may be optionally substituted poly(3,4-ethylenedioxythiophene), which has the following general structure:

Methods for forming conductive polymers, such as described above, are well known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker, et al., describes various techniques for forming substituted polythiophenes from a monomeric precursor. The monomeric precursor may, for instance, have the following structure:

wherein,

T, D, R₇, and q are defined above. Particularly suitable thiophene monomers are those in which “D” is an optionally substituted O₂ to O₃ alkylene radical. For instance, optionally substituted 3,4-alkylenedioxythiophenes may be employed that have the general structure:

wherein, R₇ and q are as defined above. In one particular embodiment, “q” is 0. One commercially suitable example of 3,4-ethylenedioxthiophene is available from Heraeus Precious Metals GmbH & Co. KG under the designation Clevios™ M. Other suitable monomers are also described in U.S. Pat. No. 5,111,327 to Blohm, et al. and U.S. Pat. No. 6,635,729 to Groenendaal, et al. Derivatives of these monomers may also be employed that are, for example, dimers or trimers of the above monomers. Higher molecular derivatives, i.e., tetramers, pentamers, etc. of the monomers are suitable for use in the present invention. The derivatives may be made up of identical or different monomer units and used in pure form and in a mixture with one another and/or with the monomers. Oxidized or reduced forms of these precursors may also be employed.

The conductive polymer may be formed in situ or pre-polymerized and then applied to the anode body in the form of a dispersion. To form an in situ polymerized layer, the monomer may be chemically polymerized, optionally in the presence of an oxidative catalyst. The oxidative catalyst typically includes a transition metal cation, such as iron(III), copper(II), chromium(VI), cerium(IV), manganese(IV), manganese(VII), or ruthenium(III) cations, and etc. A dopant may also be employed to provide excess charge to the conductive polymer and stabilize the conductivity of the polymer. The dopant typically includes an inorganic or organic anion, such as an ion of a sulfonic acid. In certain embodiments, the oxidative catalyst has both a catalytic and doping functionality in that it includes a cation (e.g., transition metal) and an anion (e.g., sulfonic acid). For example, the oxidative catalyst may be a transition metal salt that includes iron(III) cations, such as iron(III) halides (e.g., FeCl₃) or iron(III) salts of other inorganic acids, such as Fe(ClO₄)₃ or Fe₂(SO₄)₃ and the iron(III) salts of organic acids and inorganic acids comprising organic radicals. Examples of iron (Ill) salts of inorganic acids with organic radicals include, for instance, iron(III) salts of sulfuric acid monoesters of C₁ to C₂₀ alkanols (e.g., iron(III) salt of lauryl sulfate). Likewise, examples of iron(III) salts of organic acids include, for instance, iron(III) salts of C₁ to C₂₀ alkane sulfonic acids (e.g., methane, ethane, propane, butane, or dodecane sulfonic acid); iron (III) salts of aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron (III) salts of aliphatic C₁ to C₂₀ carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (III) salts of aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctane acid); iron (III) salts of aromatic sulfonic acids optionally substituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid, o-toluene sulfonic acid, p-toluene sulfonic acid, or dodecylbenzene sulfonic acid); iron (III) salts of cycloalkane sulfonic acids (e.g., camphor sulfonic acid); and so forth. Mixtures of these above-mentioned iron(III) salts may also be used. Iron(III)-p-toluene sulfonate, iron(III)-o-toluene sulfonate, and mixtures thereof, are particularly suitable. One commercially suitable example of iron(III)-p-toluene sulfonate is available from Heraeus Precious Metals GmbH & Co. KG under the designation Clevios™ C.

The oxidative catalyst and monomer may be applied either sequentially or together to initiate the polymerization reaction. Suitable application techniques for applying these components include screen-printing, dipping, electrophoretic coating, and spraying. As an example, the monomer may initially be mixed with the oxidative catalyst to form a precursor solution. Once the mixture is formed, it may be applied to the anode part and then allowed to polymerize so that a conductive coating is formed on the surface. Alternatively, the oxidative catalyst and monomer may be applied sequentially. In one embodiment, for example, the oxidative catalyst is dissolved in an organic solvent (e.g., butanol) and then applied as a solution. The anode part may then be dried to remove the solvent therefrom. Thereafter, the part may be dipped into a solution containing the monomer. Regardless, polymerization is typically performed at temperatures of from about −10° C. to about 250° C., and in some embodiments, from about 0° C. to about 200° C., depending on the oxidizing agent used and desired reaction time. Suitable polymerization techniques, such as described above, may be described in more detail in U.S. Pat. No. 7,515,396 to Biler. Still other methods for applying such conductive coating(s) may be described in U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, et al.

V. Other Layers

If desired, the capacitor may also contain other layers as is known in the art. For example, the part may be applied with a carbon layer (e.g., graphite) and silver layer, respectively. The silver coating may, for instance, act as a solderable conductor, contact layer, and/or charge collector for the capacitor and the carbon coating may limit contact of the silver coating with the solid electrolyte. Such coatings may cover some or all of the solid electrolyte.

VI. Terminations

The capacitor may also be provided with terminations, particularly when employed in surface mounting applications. For example, the capacitor may contain an anode termination to which the anode lead of the capacitor element is electrically connected and a cathode termination to which the cathode of the capacitor element is electrically connected. Any conductive material may be employed to form the terminations, such as a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin, palladium, lead, copper, aluminum, molybdenum, titanium, iron, zirconium, magnesium, and alloys thereof). Particularly suitable conductive metals include, for instance, copper, copper alloys (e.g., copper-zirconium, copper-magnesium, copper-zinc, or copper-iron), nickel, and nickel alloys (e.g., nickel-iron). The thickness of the terminations is generally selected to minimize the thickness of the capacitor. For instance, the thickness of the terminations may range from about 0.05 to about 1 millimeter, in some embodiments from about 0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2 millimeters. One exemplary conductive material is a copper-iron alloy metal plate available from Wieland (Germany). If desired, the surface of the terminations may be electroplated with nickel, silver, gold, tin, etc. as is known in the art to ensure that the final part is mountable to the circuit board. In one particular embodiment, both surfaces of the terminations are plated with nickel and silver flashes, respectively, while the mounting surface is also plated with a tin solder layer.

Referring to FIG. 1, one embodiment of an electrolytic capacitor 30 is shown that includes an anode termination 62 and a cathode termination 72 in electrical connection with a capacitor element 33. The capacitor element 33 has an upper surface 37, lower surface 39, front surface 36, and rear surface 38. Although it may be in electrical contact with any of the surfaces of the capacitor element 33, the cathode termination 72 in the illustrated embodiment is in electrical contact with the lower surface 39 and rear surface 38. More specifically, the cathode termination 72 contains a first component 73 positioned substantially perpendicular to a second component 74. The first component 73 is in electrical contact and generally parallel with the lower surface 39 of the capacitor element 33. The second component 74 is in electrical contact and generally parallel to the rear surface 38 of the capacitor element 33. Although depicted as being integral, it should be understood that these portions may alternatively be separate pieces that are connected together, either directly or via an additional conductive element (e.g., metal). Also, in certain embodiments, it should be understood that the second component 74 may be eliminated from the cathode termination 72. The anode termination 62 likewise contains a first component 63 positioned substantially perpendicular to a second component 64. The first component 63 is in electrical contact and generally parallel with the lower surface 39 of the capacitor element 33. The second component 64 contains a region 51 that carries an anode lead 16. In the illustrated embodiment, the region 51 possesses a “U-shape” for further enhancing surface contact and mechanical stability of the lead 16.

The terminations may be connected to the capacitor element using any technique known in the art. In one embodiment, for example, a lead frame may be provided that defines the cathode termination 72 and anode termination 62. To attach the electrolytic capacitor element 33 to the lead frame, a conductive adhesive may initially be applied to a surface of the cathode termination 72. The conductive adhesive may include, for instance, conductive metal particles contained with a resin composition. The metal particles may be silver, copper, gold, platinum, nickel, zinc, bismuth, etc. The resin composition may include a thermoset resin (e.g., epoxy resin), curing agent (e.g., acid anhydride), and coupling agent (e.g., silane coupling agents). Suitable conductive adhesives may be described in U.S. Patent Publication No. 2006/0038304 to Osako, et al. Any of a variety of techniques may be used to apply the conductive adhesive to the cathode termination 72. Printing techniques, for instance, may be employed due to their practical and cost-saving benefits.

A variety of methods may generally be employed to attach the terminations to the capacitor. In one embodiment, for example, the second component 64 of the anode termination 62 and the second component 74 of the cathode termination 72 are initially bent upward to the position shown in FIG. 1. Thereafter, the capacitor element 33 is positioned on the cathode termination 72 so that its lower surface 39 contacts the adhesive and the anode lead 16 is received by the upper U-shaped region 51. If desired, an insulating material (not shown), such as a plastic pad or tape, may be positioned between the lower surface 39 of the capacitor element 33 and the first component 63 of the anode termination 62 to electrically isolate the anode and cathode terminations.

The anode lead 16 is then electrically connected to the region 51 using any technique known in the art, such as mechanical welding, laser welding, conductive adhesives, etc. For example, the anode lead 16 may be welded to the anode termination 62 using a laser. Lasers generally contain resonators that include a laser medium capable of releasing photons by stimulated emission and an energy source that excites the elements of the laser medium. One type of suitable laser is one in which the laser medium consist of an aluminum and yttrium garnet (YAG), doped with neodymium (Nd). The excited particles are neodymium ions Nd³⁺. The energy source may provide continuous energy to the laser medium to emit a continuous laser beam or energy discharges to emit a pulsed laser beam. Upon electrically connecting the anode lead 16 to the anode termination 62, the conductive adhesive may then be cured. For example, a heat press may be used to apply heat and pressure to ensure that the electrolytic capacitor element 33 is adequately adhered to the cathode termination 72 by the adhesive.

Once the capacitor element is attached, the lead frame is enclosed within a resin casing, which may then be filled with silica or any other known encapsulating material. The width and length of the case may vary depending on the intended application. Suitable casings may include, for instance, “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “I”, “J”, “K”, “L”, “M”, “N”, “P”, “R”, “S”, “T”, “V”, “W”, “X”, or “Z” cases (AVX Corporation). Regardless of the case size employed, the capacitor element is encapsulated so that at least a portion of the anode and cathode terminations are exposed for mounting onto a circuit board. As shown in FIG. 1, for instance, the capacitor element 33 is encapsulated in a case 28 so that a portion of the anode termination 62 and a portion of the cathode termination 72 are exposed.

Regardless of the particular manner in which it is formed, the resulting capacitor may exhibit excellent electrical properties. The equivalent series resistance (“ESR”) may, for instance, be about 300 milliohms or less, in some embodiments about 200 milliohms or less, and in some embodiments, from about 1 to about 100 milliohms, as measured with a 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal, free of harmonics, at a frequency of 100 kHz. In addition, the leakage current, which generally refers to the current flowing from one conductor to an adjacent conductor through an insulator, can be maintained at relatively low levels. For example, the leakage current may be about 40 μA or less, in some embodiments about 25 μA or less, and in some embodiments, about 15 μA or less. The numerical value of the normalized leakage current of the capacitor may likewise be about 0.2 μA/μF*V or less, in some embodiments about 0.1 μA/μF*V or less, and in some embodiments, about 0.05 μA/μF*V or less, where μA is microamps and μF*V is the product of the capacitance and the rated voltage. The ESR and normalized leakage current values may even be maintained at relatively high temperatures. For example, the values may be maintained after reflow (e.g., for 10 seconds) at a temperature of from about 100° C. to about 350° C., and, in some embodiments from about 200° C. to about 300° C. (e.g., 240° C.).

The present invention may be better understood with reference to the following examples.

Test Procedures Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency may be 100 kHz and the temperature may be 23° C.±2° C.

Capacitance

The capacitance may be measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency may be 120 Hz and the temperature may be 23° C.±2° C.

Leakage Current:

Leakage current (“DCL”) may be measured using a leakage test set that measures leakage current at a temperature of about 25° C. and at the rated voltage (e.g., 4V) after 60 seconds.

Example 1

200,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1250° C., and pressed to a density of 5.8 g/cm³. The resulting pellets had a size of 0.76×1.22×0.67 mm. The pellets were anodized to 10.4V in a 0.1 wt. % nitric acid electrolyte for 10 hours to form the dielectric layer. To form the precoat layer, the anode part was placed in a humidified atmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dipped for 3 minutes in a solution containing manganese nitrate (specific gravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The part was placed in another humidified atmosphere (30° C., humidity of 8 g/m³) for 120 minutes, and thereafter heat-treated at 250° C. in an atmosphere having 80% relative humidity. It was determined that the resulting manganese dioxide nanoprojections had an average size of about 10 nanometers, and the surface coverage was about 10%.

Reformation was performed at 8.4V in a 0.1 wt. % acetic acid electrolyte for 20 minutes. The anode part was then dipped for 30 seconds into a solution containing 0.8 wt. % shellac and ethanol, and heat treated at 125° C. for 30 minutes. To form the conductive polymer layer, the anode part was initially dipped for 30 seconds into a solution containing 1 part of 3,4-ethylenedioxthiophene monomer, 6.4 parts of an oxidizer (50 wt. % iron p-toluenesulfonate), 6 parts of ethanol, and 1 part water. The monomer was allowed to polymerize for 60 minutes at 20° C. in an atmosphere containing 80% relative humidity, and then washed in a solution containing water, butanol, and p-toluenesulfonate (2 wt. %). Reformation was performed for 30 minutes at 8.4V in an electrolyte containing 0.01 wt. % phosphoric acid. The anode part was then dipped for 30 seconds into a solution containing an oxidizer (55 wt. % iron p-toluenesulfonate) and dried for 5 minutes at 85° C., dipped for 1 second into a solution containing 3,4-ethylenedioxthiophene monomer, and thereafter allowed to polymerize for 5 to 60 minutes at 20° C. in an atmosphere containing 80% relative humidity. The part was washed in a solution containing water, butanol, and p-toluenesulfonate (2 wt. %). The part was then dipped into a graphite dispersion and dried, and dipped into a silver dispersion and dried. The finished parts were completed by conventional assembly technology.

After testing, it was determined that the capacitance was 45.5 μF, ESR was 91 mΩ, and the leakage current was 14.6 μA (normalized leakage current was 0.051 μA/μF*V for a rated voltage of 6.3V).

Example 2

150,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1300° C., and pressed to a density of 5.8 g/cm³. The resulting pellets had a size of 1.01×1.52×0.57 mm. The pellets were anodized to 10.4V in a 0.1 wt. % nitric acid electrolyte for 10 hours to form the dielectric layer. To form the precoat layer, the anode part was placed in a humidified atmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dipped for 3 minutes in a solution containing manganese nitrate (specific gravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The part was placed in another humidified atmosphere (30° C., humidity of 8 g/m³) for 120 minutes, and thereafter heat-treated at 250° C. in an atmosphere having 80% relative humidity. It was determined that the resulting manganese dioxide nanoprojections had an average size of about 11 nanometers, and the surface coverage was about 10%. The remaining portions of the capacitor, including the conductive polymer layer, were formed as described in Example 1.

After testing, it was determined that the capacitance was 46.6 μF, ESR was 71 mΩ, and the leakage current was 12.1 μA (normalized leakage current was 0.041 μA/μF*V for a rated voltage of 6.3V).

Example 3

100,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1325° C., and pressed to a density of 6.0 g/cm³. The resulting pellets had a size of 0.70×1.08×0.57 mm. The pellets were anodized to 19.4V in a 0.1 wt. % nitric acid electrolyte for 8 hours to form the dielectric layer. To form the precoat layer, the anode part was placed in a humidified atmosphere (30° C., humidity of 8 g/m³) for 30 minutes and then dipped for 3 minutes in a solution containing manganese nitrate (specific gravity of 1.09) and 1 wt. % of a polyalkyl ether surfactant. The part was placed in another humidified atmosphere (30° C., humidity of 8 g/m³) for 120 minutes, and thereafter heat-treated at 250° C. in an atmosphere having 80% relative humidity. It was determined that the resulting manganese dioxide nanoprojections had an average size of about 11 nanometers, and the surface coverage was about 10%. Reformation was performed at 17.4V in a 0.1 wt. % acetic acid electrolyte for 20 minutes. The anode part was then dipped for 30 seconds into a solution containing 0.8 wt. % shellac and ethanol, and heat treated at 125° C. for 30 minutes. The remaining portions of the capacitor, including the conductive polymer layer, were formed as described in Example 1.

After testing, it was determined that the capacitance was 9.8 μF, ESR was 132 mΩ, and the leakage current was 0.3 μA (normalized leakage current was 0.003 pA/μF*V for a rated voltage of 10V).

These and other modifications and variations of the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims. 

What is claimed is:
 1. A solid electrolytic capacitor comprising: an anode body formed from an electrically conductive powder, wherein the powder has a specific charge of about 70,000 μF*V/g or more; a dielectric that overlies the anode body; an adhesion coating that overlies the dielectric, wherein the adhesion coating includes a discontinuous precoat layer and contains a plurality of discrete nanoprojections of a manganese oxide; and a solid electrolyte that overlies the dielectric and includes a conductive polymer layer.
 2. The solid electrolytic capacitor of claim 1, wherein the electrically conductive powder includes tantalum and the dielectric includes tantalum pentoxide.
 3. The solid electrolytic capacitor of claim 1, wherein the powder has a specific charge of from about 120,000 μF*V/g to about 250,000 μF*V/g.
 4. The solid electrolytic capacitor of claim 1, wherein the nanoprojections have an average size of from about 5 nanometers to about 500 nanometers.
 5. The solid electrolytic capacitor of claim 1, wherein about 50% or more of the nanoprojections have an average size of from about 10 nanometers to about 110 nanometers.
 6. The solid electrolytic capacitor of claim 1, wherein the surface coverage of the nanoprojections is from about 0.1% to about 40%.
 7. The solid electrolytic capacitor of claim 1, wherein the manganese oxide is manganese dioxide.
 8. The solid electrolytic capacitor of claim 1, wherein the conductive polymer layer includes a chemically polymerized conductive polymer.
 9. The solid electrolytic capacitor of claim 8, wherein the substituted polythiophene is poly(3,4-ethylenedioxythiophene).
 10. The solid electrolytic capacitor of claim 1, wherein the capacitor exhibits an ESR of about 300 milliohms or less as determined at a frequency of 100 kHz and a normalized leakage current of about 0.2 μA/μF*V or less.
 11. A method for forming a solid electrolytic capacitor, the method comprising: contacting an anode that contains an anode body and a dielectric with a solution that contains a manganese oxide precursor, wherein the anode body is formed from an electrically conductive powder having a specific charge of about 70,000 μF*V/g or more; pyrolytically converting the precursor to form a discontinuous layer containing a plurality of discrete nanoprojections of a manganese oxide; and forming a conductive polymer layer that contacts the nanoprojections and the dielectric.
 12. The method of claim 11, wherein the manganese oxide precursor is manganese nitrate.
 13. The method of claim 11, wherein the solution contains a surfactant in an amount of from about 0.01 wt. % to about 30 wt. %.
 14. The method of claim 11, further comprising contacting the anode with a humidified atmosphere prior to contact with the solution containing the manganese oxide precursor, wherein the humidified atmosphere has a humidity level of from about 1 to about 30 g/m³, a relative humidity of from about 30% to about 90%, or both.
 15. The method of claim 11, further comprising contacting the anode with a humidified atmosphere after contact with the solution containing the manganese oxide precursor but prior to pyrolytically converting the precursor, wherein the humidified atmosphere has a humidity level of from about 1 to about 30 g/m³, a relative humidity of from about 30% to about 90%, or both.
 16. The method of claim 11, wherein the precursor is pyrolytically converted in the presence of a humidified atmosphere having a humidity level of from about 1 to about 30 g/m³, a relative humidity of from about 30% to about 90%, or both.
 17. The method of claim 11, wherein the electrically conductive powder includes tantalum and the dielectric includes tantalum pentoxide.
 18. The method of claim 11, wherein the powder has a specific charge of from about 100,000 μF*V/g to about 350,000 μFV/g.
 19. The method of claim 11, wherein the conductive polymer layer is formed by chemically polymerizing a thiophene monomer. 